Conventional magnetoresistive (MR) sensors, such as those used in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin of the GMR effect is that the application of an external magnetic field causes a variation in the relative orientation of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure, called a spin valve, comprising two uncoupled ferromagnetic layers separated by a nonmagnetic metal layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the layer onto an antiferromagnetic layer which exchange-couples to the pinned layer. The unpinned layer or free ferromagnetic layer is free to rotate in the present of any small external magnetic field.
Spin valve structures have been identified in which the resistance between two uncoupled ferromagnetic layers is observed to vary as cosine of the angle between the magnetizations of the two layers and is independent of the direction of current flow. The spin valve produces a magnetoresistance that, for selected combinations of materials, is greater in magnitude than AMR. In general, the larger ΔR/R is the better the spin valve's performance.
Spin valve (GMR) read heads require two main improvements for future high density recording needs, which are larger signal for detecting ever smaller magnetic bits and smaller read gaps requiring thinner pinning layers. Most previously described spin valve use NiO as the antiferromagnetic or pinning layer deposited adjacent to the pinned layer for exchange-coupling to fix or pin the magnetization of the pinned layer. Through exchange anisotropy with the NiO antiferromagnetic layer, the magnetization of the pinned layer is held rigid against small field excitations, such as those that occur from the signal field to be sensed. However, the low magnetic anisotropy energy for NiO, less than 105 erg/cm2, led to a weak pinning field and a high critical thickness, greater than 400 Å, for pinning layer. Additionally, the low ordering temperature of 250° C. led to thermally unstable pinning.
U.S. Pat. No. 5,665,465 issued Sep. 9, 1997 to Gyorgy et al. discloses an article including a magnetically hard oxide layer in contact with a magnetically soft oxide layer, with spins in the latter at room temperature exchange-coupled to the oriented spins in the former. Both hard oxide layer and soft oxide layer consist of ferrimagnetic spinel-type oxides. However, Gyorgy et al. only teaches the existence of exchange anisotropy between two magnetic metal oxide layers, which are only in expitaxial crystalline structures. Gryorgy does not teach about the exchange anisotropy between a magnetic metal oxide layer and a metallic layer. Furthermore, Grygory does not teach or suggest the use of the exchange-coupled structures in spin valves.
An article entitled “Enhanced Blocking Temperature in NiO Spin Valves: Role of Cubic Spinel Ferrite Layer Between Pinned Layer and NiO” by R. F. C. Farrow et al. submitted to Applied Physics Letters on Jun. 19, 2000 discloses simple spin valves including an interfacial oxidized Fe layer inserted at the pinned layer/antiferromagnetic NiO layer interface to increase blocking temperature and pinning field for spin valves. The Fe-oxide layer is converted to a cubic spinel ferrite (Nio0.8Fe2.2O4) layer by solid state reaction with the NiO. Unfortunately, spin valves produced by this technique do not give larger signals for detecting smaller magnetic bits and do not have a small read gap.
There is a need, therefore, for an improved spin valve including pinning layers that overcome the above difficulties.